1. Field of the Invention
The present invention relates to a condenser lens with large aperture capable of converging with high efficiency the light emitted from a light source, to a high-efficiency light source apparatus using such a condenser lens, to a projection display apparatus using such a light source apparatus, and to a polarizing element for converting natural light with randomly oriented polarization into partially polarized light whose polarization is greater in one particular direction.
2. Description of Related Art
As a prior art example of apparatus that requires a high-efficiency light source, a projection display apparatus using liquid-crystal panels will be described below with reference to FIG. 1. The apparatus shown is disclosed, for example, in Japanese Patent Application Laid-Open No. 1-42687. In FIG. 1, indicated at 1 is a light-source apparatus having a lamp 120, a reflecting mirror 130, and a condenser lens 131. The numeral 2 indicates a beam of light radiated from the light source apparatus 1; 14B and 14R are dichroic mirrors; 11a, 11b, 11c, and 11d are mirrors; 3R, 3G, and 3B are liquid-crystal panels; 15 is a dichroic prism; 4 is a projection lens, and 5 is a screen.
Next, the operation of this apparatus will be described below. For the lamp 120 used in the light source apparatus 1, a white light source, such as a metal halide lamp, xenon lamp, or halogen lamp, is used. The lamp 120 is placed at a focal point of the condenser lens 131 which produces a parallel beam of light indicated at 2 in FIG. 1. The reflecting mirror 130 has, typically, a spherical reflecting surface, and it is known that when the reflecting mirror 130 is placed with its center of curvature positioned near the focal point of the condenser lens 131 (i.e. the position of the lamp 120), the power of the emergent light beam 2 increases by a factor of 2 or so as compared with an arrangement without the reflecting mirror 130.
The emergent light beam 2 is directed at the dichroic mirror 14B which reflects blue light and allows green and red lights to pass through and the dichroic mirror 14R which reflects red light and allows green and blue lights to pass through. The emergent light beam 2 is thus separated into three primary colors, red, green, and blue. The red light is reflected by the mirrors 11b and 11c and directed to the liquid-crystal panel 3R, while the blue light is reflected by the mirrors 11a and 11d and directed to the liquid-crystal panel 3B. The green light is directed to the liquid-crystal panel 3G. The liquid-crystal panels 3R, 3G, and 3B display color images corresponding to the three primary colors, red, green, and blue, respectively. The driving circuits used to display the images are not shown here. The beams of light modulated by the images formed on liquid-crystal panels are reunited into a single beam of light by being passed through the dichroic prism 15 of a known structure that selectively reflects the red and blue lights and selectively transmits the green light. The reunited light beam is converted by the projection lens 4 into a projection light 110 which is projected as an enlarged image onto the screen 5.
In the prior art, the converging angle .theta. (see FIG. 1) has had to be enlarged as much as possible in order to increase the illumination efficiency. A known lens arrangement that can achieve a large angle of .theta. has been such that the lens has a spherical surface, either concave, plane, or convex, on the side facing the lamp 120, and an aspherical surface of convex form on the side from which the parallel beam of light 2 emerges. This arrangement, however, has only been able to attain a maximum converging angle .theta. of the order of 40.degree. at most. An example of such a condenser lens is disclosed in "Laser and Optics Guide II," a publication by Japan Melles Griot Inc., pp. 122-124, June 1989. There has also been an attempt to construct the condenser lens 131 by combining two or three lens elements to achieve an increased converging angle .theta., but this construction has had such problems as increasing the size of the light source apparatus due to the increase in the overall length of the lens and presenting an obstacle to cost reduction because of increased complexity of lens fabrication.
When the condenser lens 131 that converts the light emitted from the lamp 120 into collimated light is constructed from a single lens, the degree of collimation of the light beam 2 emerging from the condenser lens 131 varies with the wavelength of the light. This is due to the aberration (chromatic aberration) that is caused because of the variation of refractive index of the lens material with light wavelength. If the condenser lens 131 is designed to provide an optimum degree of collimation for green light that lies in the central wavelength region in the visible spectrum, blue light on the shorter wavelength side will become a converging light while red light on the longer wavelength side will become a diverging light, resulting in different degrees of illumination for the three liquid-crystal light valves (liquid-crystal panels 3R, 3G, 3B). In the projection display apparatus of the prior art, the optical path length from the condenser lens 131 to the corresponding liquid-crystal light valves is about three times as long for the red and blue lights as for the green light. Since the red light is a diverging light and its light path length is long, the decrease in illuminance is particularly noticeable on the liquid-crystal light valve for the red light as compared to that for the green light. On the other hand, since the blue light is a converging light, color tinging will occur at the center area of the projected image on the screen 5 and the center area will appear bluish.
One known method for correcting such chromatic aberration of a single lens is to combine a convex lens, which has a small refractive index dispersion (a large Abbe number), with a concave lens which has a large refractive index dispersion (a small Abbe number). However, this lens combination has the following shortcomings.
(1) The converging angle .theta. is small because of the use of the concave lens.
(2) Since the material of low refractive index dispersion used for the convex lens generally has a low refractive index, it is difficult to design a lens that can provide a large converging angle .theta..
(3) The lens length increases, which also increases the complexity of fabrication.
Furthermore, the prior art projection display apparatus of the above construction has such problems as formation of a shadow because of the intersection of the dichroic mirrors 14R and 14B and cracking of the condenser lens 131 because of the heat of the lamp 120.
Twisted nematic (TN) liquid-crystal light valves include polarizers arranged on the front and back of a liquid-crystal layer. Polarizers have the function of allowing light vibrations in one plane to pass through, while absorbing light vibrations at right angles to this plane. Generally, plastic materials are used to form the polarizers; that is, polyvinyl alcohol films impregnated with dichroic materials such as iodine compounds or dyestuffs are oriented in one direction so as to absorb polarization component in a given direction only.
When natural light with randomly oriented polarization is incident on a liquid-crystal light valve, half of the incident light power is absorbed by the polarizers. In projection display apparatus using liquid-crystal light valves, the liquid-crystal light valves are illuminated with a high-intensity light source, so that the absorption of light by the polarizers results in the generation of heat. If the polarizers are used above their heat resisting temperature, the polarizers will be deformed and deteriorated, decreasing the degree of polarization and thus greatly degrading the resulting image quality. In particular, when plastic polarizers are used with the liquid-crystal light valves, such polarizers can only withstand heat up to 80.degree. to 90.degree. C., which presents a problem when using high-intensity light to produce images of high brightness.
An approach to resolving this problem is to remove components of light polarized in unwanted directions before the light is input to the polarizers. A prior art configuration utilizing this approach will be described with reference to FIG. 2 which shows an optical system in a projection display apparatus using liquid-crystal light valves, which is disclosed in "JAPAN DISPLAY '89 DIGEST," pp. 646-649. A lamp 120 produces a parallel beam of light by making use of a parabolic mirror 13 or the like. Ultraviolet and infrared components are removed using a filter 12, and natural light with randomly oriented polarization, collimated into a light beam 2, is converted by a polarizing element 6, called a prepolarizer, into partially polarized light whose polarization is greater in one particular direction.
The light is then separated by dichroic mirrors 7a and 7b into three color lights, red (R), green (G), and blue (B); the respective color lights are converged by condenser lenses 8r, 8g, and 8b, and then light-modulated by liquid-crystal light valves 3r, 3g, and 3b, each sandwiched by a pair of polarizers 17r/18r, 17g/18g, 17b/18b, to produce images of the three primary colors. Next, the three-primary colored images are combined by color combining dichroic mirrors 9a and 9b, and the combined color image is enlarged through a projection lens 4 for projection onto a screen (not shown).
FIG. 3 shows the structure of the polarizing element 6. As shown, the polarizing element 6 is a layered structure consisting of a plurality of glass plates 61. The arrangement of the glass plates 61 is so set as to provide the angle of incidence (the Brewster angle) at which, at the interface between the layers of air and glass having different refractive indices, all of one linearly polarized light (P polarization) 22 is transmitted while the other linearly polarized light (S polarization) 21 is partially reflected. The stacked structure of the glass plates 61 acts to produce a beam of light having the necessary direction of polarization for the liquid-crystal light valves 3r, 3g, and 3b, that is, the polarization whose direction is aligned with the polarizing axes of the incident side polarizers 17r, 17g, and 17b of the liquid-crystal light valves.
As described, in the above prior art construction, the polarizing element 6 removes the components of polarization (S polarization in FIG. 3) orthogonal to the polarizing axes of the incident side polarizers 17r, 17g, and 17b of the liquid-crystal light valves; this construction minimizes deterioration of the incident side polarizers 17r, 17g, and 17b due to heat caused by light absorption.
The polarizing element 6 of the prior art is constructed from glass plates set at the Brewster angle, typically at about 57.degree., with respect to the optical axis. If the polarizing element were constructed from a single glass plate, it would require a depth D about 1.5 times the cross sectional area of the light beam. With the one-bend construction as shown in FIG. 3, the depth D can be reduced by half. In order to increase the image brightness the projection display apparatus can produce, the lamp 120 should be placed as near to the liquid-crystal light valves 3r, 3g, and 3b as possible. In the prior art, however, the large size of the polarizing element 6 poses an obstacle to achieving this goal. Furthermore, from the viewpoint of reducing the overall size of the projection display apparatus, there is a demand for a polarizing element with reduced thickness.